The present invention relates to a redundant radio frequency (RF) network having a communication protocol for servicing roaming terminals, and more particularly, relates to a redundant communication network having a polling protocol which allows roaming terminals to selectively communicate using spread spectrum RF transmissions with host computers or with other roaming terminals through a network of many available base stations.
Radio frequency (RF) communication networks and protocols are known having remote, roaming terminals which communicate through a variety of types of communication links to host computers. Based on the overall spatial layout, response time and loading requirements of the network, different networking schemes and communication protocols have been sought so as to most efficiently regulate the communications.
Particular advantages have been identified in the use of RF communication links such as allowing remote terminals to xe2x80x9croamxe2x80x9d, free from hardwired cable connections. In basic configurations, a single host computer communicates along some hardwired link to an RF base station which would maintain an RF communication link to a single roaming terminal. As long as the roaming terminal stays within range of the RF base station and no other roaming terminals are needed, a very simple network configuration and communication protocol can be used. However, when faced with hundreds of roaming terminals which move in and out of the range of multiple RF base stations, networking and protocol problems emerge.
To solve these problems, attempts have been made to decrease the number of base stations by increasing the base stations range; however, the range of the often battery-powered roaming terminals cannot match the increased range of the wall-socket-powered RF base stations. Moreover, by increasing the range, collisions due to propagation times also increase, slowing down the overall communication time.
Other attempts have been made to increase the number of RF base stations so as to cover the entire roaming area. Although this solves the range problems associated with a single RF base station, additional problems result. First, roaming terminals which are in an overlapping range region between RF base stations communicate with one base station but receive unwanted communication from the other. Second, each roaming terminal often receives unwanted communication from other roaming terminals. Similarly, each roaming terminal often encounters xe2x80x9chiddenxe2x80x9d communications from other roaming terminals to a common base station. Because of these encounters, each roaming terminal often transmits over the xe2x80x9chiddenxe2x80x9d communications causing collisions.
Additionally, as the number of RF base stations increase, communication pathways from the source to destination become more and more complex. In a network with fixed spatial locations of base stations, host computers and remote terminals, these communication pathways from a source to a destination can easily be determined. In an environment in which the spatial layout of the network continually changes, however, determining the most efficient pathways becomes very difficult. This is because the most efficient pathway from a source to a destination continually changes due to: 1) the movement of the roaming terminals; 2) the relocation of RF base stations; and 3) the occasional break down of RF base stations and host computers.
Communication networks are also known which are often partially or completely disabled upon the break down of a single element of the network.
It is therefore an object of the present invention to provide a communication protocol between the base stations and roaming terminals for optimizing the utilization of the RF range of each base station.
It is a further object of the present invention to provide an adaptive communication network with inherent redundancy.
It is another object of the present invention to provide a communication protocol for use in a network of host computers, base stations and roaming terminals which is not susceptible to collisions with xe2x80x9chiddenxe2x80x9d communications.
It is yet another object of the present invention to provide a communication protocol which minimizes collisions in the overlapping regions of different RF base stations.
These and other objects are achieved in a method of beginning a data exchange over an RF communication link. The sending device initially identifies the fact that the RF communication link is clear during a period at least as long as the maximum interpoll gap. Thereafter, a request for poll frame is transmitted by the sending device.
In addition, these and other objects are achieved in a method used by a remote terminal having an RF range for selectively attaching itself to one of a plurality of RF base stations. Each of these base stations has an associated cost, a preset priority, and a preset number. The remote terminal receives a message from each base station and discards those which fall below a predetermined minimum threshold level. The remote terminal will attach itself to one of the plurality of base stations based on the cost, signal strength, preset priority, and preset number.
Additionally, these and other objects are achieved in a method for selecting and redundantly replacing a root device when it breaks down from among a plurality of potential root devices. Each of the potential root devices has a single, assigned preset number. The potential root device with the lowest assigned preset number is initially selected. Whenever the selected root device breaks down, one of the potential root devices will be selected based on the lowest assigned preset number without considering the preset number of the currently selected root device.